The invention relates to an arrangement for connecting a module to a switched-on supply voltage in the case of an apparatus of modular construction, in particular a computer.
Many electric apparatuses are nowadays of modular construction, that is to say they comprise a plurality of functional units which are constructed as self-contained modules and can be exchanged easily and individually, an apparatus functioning fully only once these functional units cooperate inside it.
By contrast with a monolithic structure of "one piece", or a plurality of parts firmly connected to one another, the construction of an apparatus using modular technology exhibits a range of advantages. Thus, in the case of a defect, for example, it is not necessary to exchange the entire apparatus or at least large parts thereof, it being sufficient instead to replace only the defective module. Since, as a rule, the individual modules are installed in the apparatus via plug-in, latching or screwed connections, it is therefore possible for an exchange to be carried out quickly and simply, that is to say frequently also by a person not skilled in the art. Relevant examples here are the "plug-in cards" (graphics cards, sound cards, network cards etc.) or memory modules which are installed in so called slots in a personal computer and can mostly be installed or exchanged by the users themselves.
Some apparatuses often need to provide particularly high operational reliability, that is to say protection against partial, or even complete, failure. This is the case, inter alia, for telephone exchanges, security systems and also in computers. If, for example, the computer of a retail company or of a mail order firm fails for just one day, it is not usually possible to invoice on that day, that is to say payments both in and out are blocked and the company is therefore unable to pay for this period. Such a situation could result in serious consequences for many companies.
The required operational reliability of such apparatuses is mostly achieved by all the important functional units of the apparatus being constructed as easily exchangeable modules which are best also redundant, that is to say a number of them are present as reserves. If a defect then occurs in a module, an automatic switchover is made to one of the reserve modules without the need to interrupt operation of the apparatus for this purpose or to restrict the scope of operation. If the defective module is not present in a redundant fashion, the apparatus initially likewise continues to remain in operation as far as possible, but without in this case having recourse to the functions of the module to be exchanged. As a rule, a defective module is indicated by the apparatus, so that it can be repaired or replaced by a new module. In this case, the defective module is removed and the repaired or new module is installed in a "live" fashion, that is to say while the apparatus and the terminal of the module therein continue to be supplied with voltage. In order in this case to avoid malfunctions, for example loss of data in a computer, it is indeed necessary as a rule to shut down the operational environment of the module to be exchanged, that is to say the system bus, for example, for the short duration of the exchange, as a result of which the apparatus is only operational to a limited extent during this time. However, the system (apparatus) as a whole need not be switched off, but can, for example, remain on standby. For this reason use is made of the terms "Online Replacement (OLR)" and "Live Insertion" of modules. The abbreviation "OLR" is used below for these.
the supply currents possible in this case immediately after the (re)installation of a module frequently greatly exceed the strength of the currents absorbed by the module in normal operation. The chief reason for this is that it is usually necessary first of all for capacitors (for example stabilization capacitors) to be charged up in the module before the current absorbed is reduced to the amount which is normal for operating the module. These short term current peaks occurring during installation of the module are problematic if the module itself or other parts of the apparatus can be damaged thereby, or if the voltage supply of other parts/modules of the apparatus thereby drops briefly to such an extent that correct functioning thereof is limited or even becomes impossible (some components, for example memories or counters, react to a drop in their supply voltage below a specific threshold value by resetting and thus losing data).
In order to avoid excessively high currents during online replacement of modules, the supply inputs of the appropriate modules are mostly protected in practice by current-limiting components, for example ohmic series resistors. The higher the maximum permissible current strength turns out to be in this case, the more space is usually required by the current-limiting components in the module. In the case of powerful computers, the maximum current strength may be, for example, .about.100A or more, the result being that the OLR current limitation requires a substantial amount of space in the modules for the components which are to be dimensioned correspondingly. However, this limits the packing density of a plurality of modules in the apparatus. p Laid-open patent application DE 34 02 845 A1 discloses a plug-in module with leading contacts, in which a series resistor is provided on the module for current limitation.
The publication "Taking the zap out of hot plugging" by J. D. Herard, Machine Design, Vol. 63, No. 24, pages 46, 48, 50, 54 dated Nov. 22, 1990 provides a general treatment of the problem of current impulses in the case of online replacement and generally proposes leading contacts and devices for current limitation, without specifying concrete embodiments.
Document DE-A-3 926 352 shows a device for current limitation, without the latter being bridged by a contact element.